Electrical heating appliances, and particularly electrical heating apparatus in the form of bedcovers such as electric blankets, are popular with consumers and have represented a commercially significant area of development. In part for those reasons, it is significant, in such apparatus, to provide for protection against an overheating condition. Many arrangements for protecting against an overheating condition are known from previous work and at least some of such arrangements have achieved significant commercial success.
Due in part to the popularity of such electrical heating apparatus with consumers, agencies which certify the safety of such apparatus insist upon the use of electrical circuitry which is effective for protecting a user even in the event of failure of a component of the circuitry.
While "solid state" or semiconductor circuits such as those illustrated in prior U.S. Pat. Nos. 3,356,825, 3,588,446 and 4,198,562 have achieved some success, attainment of a desired result of both a smooth bedcover structure and failsafe circuitry has presented some difficulty. More particularly, approval by certifying agencies is denied where failure of any given circuit component can possibly result in the electrical heating apparatus failing with electrical current applied to an electrical heating element, due to the risk of an occurrence of overheating.